The invention relates to a voltage-current converter comprising, between a first and a second supply terminal, a first current source coupled at one end by means of a first transistor arranged as a diode to a first terminal for conveying a reference potential and at the other end by means of a second transistor arranged as a diode to a second terminal. A series arrangement of an input port for applying a signal voltage and a first resistor for converting the signal voltage into a signal current are provided between the first and the second terminal, the converter further comprises third and fourth transistors whose bases are coupled to the first and the second terminal, respectively, the emitters being connected by means of a common second current source to the second supply terminal and the collector of at least one of the third and fourth transistors constituting an output for supplying an output current which is proportional to the signal current. The converter also comprising a correction circuit for correcting the non-linearity in the voltage-current conversion due to the non-linear emitter resistors of the first and second transistors.
A voltage-current converter of this type can be generally used and is particularly suitable for use in a device for measuring the effective value of a signal voltage.
A voltage-current converter of this type is known from U.S. Pat. No. 4,268,759. In this converter the first and the second transistor convey the same bias current. Since the first terminal is connected to ground, the second terminal constitutes a virtual ground. The signal voltage which is converted into a signal current across the first resistor is superposed on the bias currents of the first and second transistors. Together with the first and the second transistor, the third and the fourth transistor constitute a so-called translinear circuit in which the signal currents of the first and the second transistor are transferred in an amplified manner to the third and fourth resistor.
In this converter the non-linear emitter resistors of the first and the second transistor produce a linearity error in the voltage-current conversion. In order to correct this error the known converter comprises a correction circuit which is constituted by two transistors having their bases and collectors cross-coupled and in which one transistor is connected between the emitter of the first transistor and the first terminal and the other transistor is connected between the emitter of the second transistor and the second terminal.
A drawback of this correction circuit is, however, that at relatively high frequencies of the order of several tens of MHz it has a resonant rise effect causing errors in the voltage-current conversion.